The invention relates to a protective device for an electric load according to the preamble of claim 1.
Electric tools such as, e.g. hand drills are frequently constructed with double protective insulation, i.e. that, apart from a primary insulation in the motor itself, a further second insulation, e.g. in the form of an insulating housing, is provided. However, electric machines are also known in which double protective insulation is not possible because, e.g. the external housing must consist of metal. To ensure the required safety for the operator in these cases, so-called fault current circuit breakers like, e.g. generally known ground-fault circuit interrupters are frequently provided which are capable of detecting a fault current and, if necessary, of immediately interrupting the current supply to the electric device. In some countries, there is, therefore, a rule to provide such circuit breakers in switchgear cabinets and distribution cabinets. In most countries, however, there is no such rule.
The resultant problems can be partially resolved by a protective device (safety interrupter PRCD-S by KOPP) by means of which a fault current circuit breaker can be built into the feed line to the load.
Furthermore, electric devices are known in which the electric energy supplied from the outside via a mains connection can be temporarily stored in an energy store, e.g. a capacitor which can be, e.g. a component of a frequency converter, and is forwarded only with a time delay. In such devices, the protective effect of the fault current circuit breaker can fail since the energy stores are often equipped with such a high capacity that they still store sufficient energy to put the operator seriously at risk in spite of the current supply from the outside being interrupted. This means that a fully effective protection of the operator can not always be guaranteed even when the fault current circuit breaker is provided in the distribution cabinet.
The invention is based on the object of specifying a protective device for an electric load which effectively prevents fault currents from arising even when there is no corresponding protective device in the power system or, respectively, the device exhibiting the electric load is provided with an energy store in which the current supply by the mains is temporarily stored.
According to the invention, the object is achieved by a protective device having the features of claim 1. Advantageous further developments of the invention can be found in the dependent claims.
In the protective device according to the invention, a fault current circuit breaker is arranged between the mains connection and a switching device in a feed line connecting a mains connection to an electric load. The fault current circuit breaker exhibits a detection device for detecting a fault current and a first interrupter for interrupting a current supply from the mains connection to the switching device in dependence on a switching signal from the detection device. Furthermore, a second interrupter which can be switched by the switching signal from the detection device is provided downstream of the first interrupter closer to the load. The first interrupter is advantageously arranged in such a manner that it can interrupt the current supply from the mains connection to the fault current circuit breaker. Since the second interrupter is arranged closer to the load, it protects the latter, and the operator, better than is possible by means of the first interrupter alone. This applies, in particular, if the second interrupter isolates the current supply electronically.
By arranging the fault current circuit breaker in the area of the current feed line to the electric device or, respectively, directly at the mains plug in this manner, it can immediately deenergize the electric load, and thus the entire electric device, when a fault current is detected. As a result, almost the entire device can be protected against fault currents even if there is no fault current circuit breaker in the distribution cabinets belonging to the power system.
Providing the second interrupter is particularly suitable for electric devices in which an energy store is provided for temporarily storing electric energy in the area of the switching device, e.g. as a component of a frequency converter. As already explained above, there is a risk with such energy stores that they can still store enough energy to put the operator at risk due to fault currents in spite of an interrupted mains current supply. This problem can still exist even if the first interrupter has already been activated.
The energy store is arranged, e.g. between the fault current circuit breaker and the switching device. Any current flow to or from the energy store can be interrupted by the second interrupter. The detection device is thus capable of activating by means of the switching signal not only the first interrupter which disconnects the electric device from the mains but also the second interrupter which should be arranged as closely as possible to the energy store. If a fault current occurs in this case, e.g. at the electric load, the connection to the mains, the connection to the energy store and the connection of the energy store to the output of the power section (e.g. the output stage of a frequency converter) is interrupted. The energy stored in the energy store can no longer escape which eliminates any risk to the operator.
In a further advantageous embodiment of the invention, the first and/or the second interrupter are closed only when the switching signal is present. If there is no switching signal, the electric connection is broken. This ensures that the fault current circuit breaker is operating correctly and has not been removed from the device, either. It is impossible to put the device into operation when the fault current circuit breaker is missing.
In the text which follows, these and other advantages and features of the invention will be explained in greater detail by means of an example and with the aid of the accompanying FIGURE.